Ink for ball point pens must be especially compounded for such unique usage. It differs greatly from pigmented printing inks on the one hand, and from aqueous fountain pen inks on the other hand. Thus it must have the required permanence of the color ingredient or dye, usually employed in a non-aqueous carrier, plus the necessary viscosity and flow characteristics for passing through the small clearance between the ball and its end socket. It must also be quick drying and non-spreading or non-smearing. Carriers which bleed or strike through to the reverse face of the writing paper must be eliminated.
The industry has succeeded in improving the writing quality of ball pen inks to a point at which they are far superior to earlier inks. However, one important and unresolved problem remains. Even these relatively good inks degrade with the passage of time and finally refuse to flow from the cartridge. This is the problem of shelf life or aging of the writing fluid. Aging occurs during the initial period of distribution to retail outlets in which it is shipped and sometimes stored in humid environments. Aging occurs on the shelves of the retailer. Finally it occurs during the period of usage by the final purchaser. Each of these periods may extend over many months. Ideally as long as there is any fluid in the supply cartridge or reservoir, the pen should be operative without hesitation or faltering.
Preserving the functional quality of an initially satisfactory formulation of ball point pen ink stored in a plastic cartridge is a very different problem from preserving its character in a sealed air-tight bulk container. Comparatively small quantities or "charges" of ball point pen ink are placed in small diameter tubes, commonly of polyethylene, polypropylene, or other plastic, and one end of which is open to the atmosphere. The other end is closed by a metal nib terminally carrying a ball rotatable in its socket. The open end of some larger diameter cartridges, too large for the ink to be retained by capillary action, may be plugged by a so-called "grease follower" or other type of "plug" which is not 100% impervious to oxygen and water and which moves into the cartridge as the ink flows out. During storage and use the ink of the cartridge is subject to the effects of contact with the atmosphere at this end even though closed by a plug.
Great quantities of these cartridge units including a writing nib are machine assembled, transported all over the world, and stored under all kinds of conditions. When purchased, or before being placed in use, a cartridge is simply thrust inside a tubular holder to provide a complete and instantly usable writing assembly. Such a newly assembled pen should function until the ink is entirely drained from the cartridge. If it does not, and today frequently it does not if the cartridge has aged for 1 to 3 years or less, under variable climatic conditions, the purchaser understandably is dissatisfied.
Examination of an aged disassembled pen which has failed to write has repeatedly shown what is known as "crystallization", that is, at the back or inner end of the bronze or brass tip which extends into the plastic body of the cartridge, a crystalline cap-shaped mass has accumulated which blocks the outlet passage going down the nib through which the ink flows to the writing ball at the point. This chokes off the continuous flow of ink and makes writing impossible. When this occurs, there is no practical step the user can take to reverse the process, and the entire cartridge or writing instrument is simply discarded. What formed this crystalline mass or cap had not hitherto been known. It did not form at this location in brass cartridges. It did not form there in the ink not in plastic cartridges. In brass cartridges usually of larger diameter and provided with a movable plug or closure at the open end spaced from the nib, the effect was noted not at the nib but at the interface of the plug and the ink. This effect caused the plug to freeze so that it could not follow the ink as its level approached the nib. The result was a vacuum which prevented the ink from flowing through the point.
Laboratory studies have now disclosed that the crystallization forms a heterogeneous mass of crystals known as a "crystal cap" through which ink cannot penetrate and which is characterized as follows:
1. This process is gradual; PA1 2. During the early stages of crystal formation and before a cap is formed, the phenomena can account for starvation and skipping caused by larger crystals; PA1 3. The crystalline mass is composed of metal salts and precipitated ink components and corrosion by-products;
a. The consistency of the mixture resembles common asphalt in texture, viscosity and color. PA2 b. It can be several hundred microns to several millimeters in depth depending on the time and the particular ink formulation from which it was formed. PA2 c. A confirmation of the metallic content of the mass made by atomic absorption spectroscopy discloses a concentration of Cu++ and Zn++ approximately 300 times the normal concentration of these metals present elsewhere in the ink.
It has been observed that fountain pen inks, for example, may deposit sediment upon standing in an open container or pen due to evaporation of liquid which results in increased viscosity of the fluid. On the theory that something like this may have occurred in ball point pen ink, (1) volatile components were eliminated from the carrier, and (2) more impervious plastic cartridges were tried. The plastic body was even impregnated with metal flakes to prevent passage of liquid or gaseous components out through the walls. Those steps did not appreciably increase the storage stability. On aging, the resultant ink still produced gummy and/or crystalline particles which eventually choked off the flow.
Examination of many cartridges relative to the phenomenon of crystallization and crystal caps disclosed that the phenomenon did not occur with a nickel or stainless steel point but only with a point of brass, or other copper alloy such as bronze. It did not occur at the point when using a brass cartridge, but only in a plastic cartridge. It took place with various kinds of ball pen inks made by many manufacturers, the only significant difference being a variation in the length of time it took to develop a crystal cap in various of the ink formulas.
Until the present development all effort to eliminate the flow-blocking mass has failed to the continuous irritation of both users and manufacturers, such difficulty causing serious inventory and quality control problems for the latter.